Light-emitting diodes are a permanent component in modern electronics. They permit minaturization and, due to their excellent mechanical stability, low operating temperature, long life and compatibility with semiconductor circuits, find application, for example, as indicator lamps, displays, infrared emitters, opto-couplers and infrared light gates. Luminescent diodes are divided into two classes, i.e., those which emit in the visible spectral range and those which emit in the infrared spectral range.
For luminescence in the visible spectral range, the Group III-V compounds GaP and GaAs.sub.1-x P.sub.x mixed crystals have become particularly important. Depending upon the composition and the doping, wavelengths between red and yellow-green (690 to 570 nm) can be adjusted.
Ga.sub.1-x Al.sub.x As mixed crystals can be used in the infrared spectral range below 900 nm and as red light diodes in the visible spectral range (J.Appl.Phys. 48 (1977), pages 2485 to 2492). They make it possible to adjust any desired wavelength of the light emission between 650 and 900 nm. Besides GaAs, Ga.sub.1-x Al.sub.x As:Si mixed crystals can be used to advantage for red emission as well as for luminescence in the infrared spectral range.
The preparation of epitaxial Ga.sub.1-x Al.sub.x As films via the liquid-phase epitaxy is known (J.Appl.Phys. 48 (1977), pages 2485 to 2492). The layer growth is accomplished by cooling a Ga, Al melt which is in contact with a GaAs substrate and is saturated with As, starting from a constant temperature. The p-n junction required for the construction of luminescent diodes is prepared during the growth of an epitaxial layer via incorporation of the doping substance Si. Due to the incorporation of the Si atoms in Ga-lattice locations (which predominates at higher temperatures), the n-conduction layer grows first. The p-conduction layer then follows due to the predominant incorporation of the Si atoms in As-lattice locations at lower temperatures. By varying the mol content x AlAs in the mixed crystal Ga.sub.1x Al.sub.x As, the energy bandwith, and thereby, the wavelength of the light emission, is changed.
In these methods, so-called push boats of graphite are used to receive the Ga, Al, As-melts as well as the GaAs-substrate wafers to be coated (J.Appl.Phys. 48 (1977), page 2485, particularly FIG. 3). Besides the extremely complicated construction of the known push boats, the capacity of substrate area to be coated per coating operation is low. It is thus necessary to use single-crystal substrate wafers of uniform geometry. Because of the thickness of Ga melts which are specifically related to the use of boats, the homogenizing times are several hours.